I just finished playing around with 1Writer and iA Writer myself.
iA Writer has been my editor of choice for years. Sadly, iA Writer’s implementation of transclusion is its own proprietary approach: content blocks. Content blocks are cool, but they don’t follow the same syntax as Obsidian, so a file transcluded in iA Writer won’t work properly in Obsidian and vice-versa.
1Writer follows the same [[double square bracket]] syntax as Obsidian for transclusion, so it is a great way to view and traverse your Obsidian vault on an iPhone or iPad. Of course, you can also edit documents in your vault using 1Writer. Given that 1Writer supports [[double square bracket wiki linking]], I would’ve thought it would offer some kind of autocomplete when you enter [[—but no. So, you’ll have to go look up the exact title of existing notes in order to properly link to them in a new edit to a document in 1Writer. (Of course, there’s also no way to see backlinks.)
For now, 1Writer wins out. Still, if an app exists that offers autocomplete upon [[-ing, I’d love to know about it.